There are two types of projection-type image display devices (also described as a projector hereinafter). One is a liquid crystal projector that provides a projection image through transmitted light from a liquid crystal display element. The other is a DLP (Digital Light Processing, registered trademark) projector that provides a projection image through reflected light from a reflective mirror device element such as a DMD (digital micro-mirror device) element. A projector of either type decomposes white light from a white-light source into the three primary colors (red: R, green: G, blue: B) with an optical device such as a prism; illuminates a display element corresponding to each color with each light; and modulates the illuminated light with a video signal to produce a projection image. Particularly, DLP projectors are usually used for business use such as in a conference hall and theater where high luminance and high reliability are demanded. For a white-light source used for a projector, a discharge lamp with high luminance is often used such as a high-pressure mercury lamp and xenon lamp.
In a DLP projector, three-primary lights are illuminated on DMD elements corresponding to each pixel of a projection image; output light to the projection lens is controlled according to changes of the deflection angle to provide a projection image.
A type of DLP projector incorporates three pieces of independent DMD elements each corresponding to three-primary lights (R, G, B); however, the type is complicated while producing high image quality, and is expensive due to the high price of DMD elements. Under the circumstances, a method has become widely used for low-cost projectors that illuminate DMD elements with three-primary lights sequentially and time-divisionally.
To produce three primary colors, a disc (called a color wheel) on which flat surface three pieces of optical filters (segment) each transmitting each of three primary colors are arranged is rotated orthogonally to the light axis of a discharge lamp (as a white-light source) to produce three primary colors sequentially and time-divisionally from white light of a discharge lamp. To increase the luminance of a projection image, a color wheel with 4-color segments including a white filter can be used.
If a discharge lamp is driven by DC power, electrons collide with the positive electrode in arc discharge, thereby increasing its temperature higher than the negative electrode, which consequently shortens the life of the lamp. Hence, the electrode pair is applied with AC power of a predetermined frequency to drive the lamp in a periodically alternating manner between the positive and negative electrodes.
The color wheel is synchronized with the cycle (e.g. 1/60 second in NTSC method) of one-frame image and is rotationally controlled at the same cycle or one over an integer of the cycle.
The aforementioned alternating cycle of a discharge lamp is desirably less than 1/100 second because alternating polarity causes discontinuity of luminance, thereby deteriorating the image quality due to flicker or other phenomena. Meanwhile, when polarity changes in a segment region of the color wheel, an instantaneous interruption, overshoot, ringing, or other phenomena of lighting may occur to deteriorate the image quality. Accordingly, control is exercised changing the polarity at a boundary between each segment of the color wheel.
FIG. 2 schematically shows a configuration of a typical color wheel with four colors. As shown in FIG. 2, the center angles (area size) of the 4-color, fan-shaped segments are not equal to each other. In the example of FIG. 2, each angle of G (green), W (white), and B (blue) segments is equal to the other while the angle of R (red) segment is set wide. In other words, the color wheel is structured so that the ratio of light amount transmitting through the color wheel when the color wheel makes one rotation varies between R, G, B, and W. The ratio is determined in consideration of the spectrum distribution of emission energy from the discharge lamp, the color sensitivity of the human eye, and others. The ratio naturally varies also depending on a light source used.
In such a 4-color (even-numbered colors) color wheel, the polarity of the discharge lamp always remains unchanged (the voltage remains positive, for instance) in the period of R (red) with a large angle of the segment in FIG. 2 for instance, when the polarity of the discharge lamp is changed at a boundary between segments. Hence, energy concentrates at one side of the electrode pair to promote deterioration of the discharge lamp. Meanwhile, in a 3-color (odd-numbered colors) color wheel (not shown), the polarity of the discharge lamp is inverted every time the color wheel makes one rotation. Hence, such a problem does not occur even if the ratio of light amount transmitting through the color wheel varies between R, G, and B.
As a method of placing equal burdens without concentrating energy on one electrode, a method shown by patent literature 1 has been devised, for instance. In the method of patent literature 1, changing the polarity inversion is periodically omitted odd number of times in a method of changing the polarity of a discharge lamp at a boundary between the 4-color (even-numbered colors) segments of a color wheel.
This method is described in the example of FIG. 2 for instance. That is, if the polarity of the electrodes is not changed at the boundary between the W and B segments, the polarity of the R segment inverts from the previous polarity, thereby appropriating imbalance in polarity.
The method of patent literature 1, however, omits changing the polarity inversion intermittently to appropriate imbalance in discharge polarity, which may cause the quality of a projection image to deteriorate due to a phenomenon such as flicker involved in polarity inversion depending on the cycle and period of omitting the changing action.
[Prior Art Document]
[Patent Literature]
    [Patent literature 1] Japanese Patent Unexamined Publication No. 2008-146837